p=6;t0=400+273; v=0.1; nsum0=p*1000000*v/(8.314*t0);
x0H2=0.58;x0CO=0.06; x0CO2=0.09; x0H2O=0.27;
n0H2=x0H2*nsum0;
n0CO=x0CO*nsum0;
n0CO2=x0CO2*nsum0;
n0H2O=x0H2O*nsum0;
mcat=0.008;

nsum=(nH2[t]+nCO[t]+nCO2[t]+nH2O[t]);
pH2=nH2[t]/nsum*p;
pCO=nCO[t]/nsum*p;
pCO2=nCO2[t]/nsum*p;
pH2O=nH2O[t]/nsum*p;

kb=1.16*10000000000*Exp[-126573/(8.314*t0)];
kco2=1.02*0.000001*Exp[67439/(8.314*t0)];
kpb=10^(-2090/t0-2.018);
kH2=2.71*100*Exp[-6291/(8.314*t0)];
kH2O=3.8*0.0000000001*Exp[80876/(8.314*t0)];
rate=-kb*kco2*(pCO2*pH2-pCO*pH2O/kpb)/((1+kco2*pCO2)*(1+kH2^0.5*pH2^0.5+kH2O*pH2O))

NDS=NDSolve[
{nH2'[t]==mcat*(-rate),nCO2'[t]==mcat*(-rate),nCO'[t]==mcat*(rate),nH2O'[t]==mcat*(rate),
nH2[0]==n0H2,nCO2[0]==n0CO2,nCO[0]==n0CO,nH2O[0]==n0H2O},
{nH2[t],nCO2[t],nCO[t],nH2O[t]},
{t,0,10},MaxStepSize->0.01]

Plot[rate/. NDS,{t,0,0.1}]
Plot[nH2[t]/nsum/. NDS,{t,0,0.1}]

Table[rate/.NDS,{t,0,1,0.1}]